Ocean Stress Project

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Determination of the direction and magnitude of stresses within the oceanic crust to constrain models for the forces driving the plates is a principal objective of the next ten years of the Ocean Drilling Program. Although observations of stress-induced wellbore breakouts with ultrasonic televiewer wellbore scanning have provided indications of the directions of the maximum and minimum principal horizontal stresses in these ODP boreholes, no direct measurements or estimates of stress magnitudes have been made. Recent theoretical developments applied to sites on land can be extended to holes drilled in the oceanic crust to allow constraints to be placed on the magnitudes of in situ stresses based on the presence or absence of breakouts and other wellbore failure phenomena, and from constraints imposed by the frictional strength of the crust. This project will estimate magnitudes of in situ stress by re- analyzing existing borehole televiewer data. Breakouts will form when the difference between the circumferential stress concentration and the radial stress at the borehole wall exceeds the strength the rock. Constraints can be placed on the stress magnitudes if breakouts occur and if the strength of the rock is known. Corrections will be made for thermal effects and pore pressure effects. Laboratory measurements will be made of the tensile and unconfined compressive strength for rock from the depth at which observations of tensile cracks or breakouts are obtained.